I've found a couple cool low pressure high oxygen worlds, RS 8474-253-8-15053162-512 3, RSC 8474-1192-4-2250-24 3, RS 8474-1792-8-9448635-211 4, and HIP 11337 5.4. Although I am using a couple visual mods, a bunch of ship mods, and a couple planet addition mod, but I don't think those would change life and atmoshpere characteristics of generated planets, but I what do I know.

I've found a couple cool low pressure high oxygen worlds, RS 8474-253-8-15053162-512 3, RSC 8474-1192-4-2250-24 3, RS 8474-1792-8-9448635-211 4, and HIP 11337 5.4. Although I am using a couple visual mods, a bunch of ship mods, and a couple planet addition mod, but I don't think those would change life and atmoshpere characteristics of generated planets, but I what do I know.

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Except for the third one, the pressures on these planets would be far too low to be livable for humans. The third one definitely has enough oxygen perhaps too much, but the concentrations of CO2 and H2S would be problematic.

Here is the full list of requirements:Pressure above 0.3 atmospheresBetween 0.1 and 0.8 bars of oxygenless than 0.01 atmospheres of CO2Temperature between -80 and 110 FahrenheitLess than 5 ppm of SO2Less than 50 ppm of H2SStar mass above 0.8 Solar masses (anything less massive would have atmosphere-destroying flare-ups in real life), below 2 Solar massesSystem not located in a galactic core or cluster(open or globular)System is not multiple-star

Free oxygen is so reactive that it doesn’t last long in the atmosphere unless plants and other photosynthetic organisms continuously replenish it. This means that on any planet, including Earth, life must initially evolve without any oxygen to breathe.

There are lots of modern bacteria that ‘breathe’ many other elements and compounds, including sulphur, carbon dioxide, iron, manganese, cobalt and uranium. All of these are much less common in the Universe than oxygen though, which is the third most common element.

The reason we breathe in oxygen is because it’s a highly reactive molecule that can combine with the compounds in our food to build new molecules and release energy. Other metabolic pathways can achieve a similar result, albeit less efficiently, using nitrate or sulphate instead of oxygen, but they all share the chemical property of being oxidizing agents. Carbon dioxide (CO2) is already fully oxidized, so it can’t be used to power respiration in the same way.

Of course, plants take in CO2 for photosynthesis, but this is a completely separate metabolic process from breathing. Plants make their food using photosynthesis, but they still need to breathe in oxygen to ‘burn’ this food. So the CO2 they absorb doesn’t count as breathing, but even if it did, it’s doubtful that a photosynthetic organism would ever evolve intelligence. Brains are metabolically very costly organs and photosynthesis doesn’t generate energy fast enough to run on.

At the bottom of the Mediterranean Sea, in one of the most extreme environments on Earth, there is evidence of an animal able to live its entire life without oxygen.

Loriciferans are about the size of a large amoeba. They live in muddy sediments at the bottom of the seas. But supposedly, that mud should contain some oxygen to allow the animals to breathe. The mud in the L'Atalante basin at the bottom of the Mediterranean does not.

Also, creatures that can make their own food just by pointing at the Sun don’t need to hunt, plan or even move much. So there’s little selective pressure for them to evolve intelligence in the first place.

A concept presented about life that used different forms of respiration involved a world where you stored oxidizers for fuel, and breathed in reducers (like hydrogen) instead of our current process of storing reducers for fuel (like fats and sugars), and breathing in oxidizes. However, these aliens might combust when exposed to our atmosphere.

Also, rather than water, using Ammonia as a solvent would work, though if you want it to be pure ammonia, you'll have to keep your aliens quite cold, as it boils at -33°C. Aqueous ammonia solutions would work, but that does not seem to be what we're after. Ammonia isn't as broadly useful a solvent as water, but it does allow for lots of interesting chemistry to occur, including things like relatively stable solutions of solvated electrons, which are quite unstable in aqueous systems. Lots of biological reactions will produce water, so an anhydrous alien will need a way to excrete the water just as we have a way to excrete ammonia. Maybe our alien could excrete crystals of glycerol or urinate an alcohol or something like that. In any case, ammonia is a much better choice than a hydrocarbon.

Breathing hydrogen is a bit more problematic. The purpose of breathing oxygen is that we can break C-H bonds and combine oxygen with both carbon and hydrogen atoms to release energy in the metabolism of organic molecules. Oxygen is highly reactive stuff; we just don't realize it because we've evolved to walk around at the bottom of an ocean of it. If we breathed hydrogen instead, it would be far less reactive. Hydrogen will readily react with lots of things, but it typically doesn't release huge amounts of energy in the process.

One idea might be to have our aliens run a biological version of the Sabatier process. The reaction is spontaneous at low temperatures, though it would never actually occur without catalysis at liquid ammonia temperatures. This would give them a means of producing some basic metabolic building blocks from materials that are common in the universe as well as provide a reason for them to breathe hydrogen in the first place.

Water would be a waste product and not necessarily a toxin (although it might be a toxin). Sort of like how CO2 is for humans. If this was a low temperature environment, the critters would excrete solid water though - imagine pooping ice cubes! I was also thinking that it might make more sense for the critters to breath methane than hydrogen.

Ammonia can work as a solvent in place of water or in addition to water. Hydrogen is also very reactive so it could be a useful energy source. Also if a planet somehow had high concentrations of Boron then Boron Nitride could be a substitute for Carbon as together they form the same bonds as Carbon and life based on Boron and Nitrogen might use ammonia as a solvent as at the temperatures of liquid ammonia that reactions between Boron and Nitrogen could be more controlled. In this case the planet might have an atmosphere of Diborane, Hydrogen, Nitrogen, and Ammonia vapor. In this case the equivalent of plant life might get energy by reacting diborane and ammonia to produce sugar analogues and hydrogen while the equivalent of animals might react hydrogen and sugar analogues to produce diborane and ammonia. In this case the gasses that the organisms would breath would explode when in contact with air. Also liquid water in high concentrations would be too hot and too acidic for this type of life and if this life touched life on Earth both would mutually poison each other!

Chlorine: The problem with chlorine is that it is very reactive and nearly impossible to find freely in the atmosphere. However, you can have chlorine compounds (HCl, CFCs etc) in the atmosphere which the creature can inhale and absorb these compounds through lungs and then use solar-cell like organs to electrolyse them and release the chlorine. They would then use this chlorine and use it in their respiration.

Self sufficient salts: This could be an interesting choice, considering that you obtaining everything from one food source. Our creatures would be eating salts (NaCl, KCl etc) and then using the above solar-cell like organs to electrolyse them in water-solution and release hydrogen and chlorine. They would absorb these gases in different chambers and transport them to the organs in different channels. Within the organs, the hydrogen and chlorine would be allowed to mix, releasing energy. The waste product (aka HCl) would then be excreted out of the organism.

Chemistry homework:Reaction

CH4 + H2O + light -> [sugar] + 2H2

is possible and requires 4 times less energy than normal photosynthesis.

Therefore, the reverse reaction of reducing sugars with hydrogen should be exothermic and yield 1/4 the energy - probably not enough for warm-blooded creatures, but surely enough for muscles and nerves to function.

I see people say that too much CO2 and SO2 kill humans. Ok. But what about alien life, the native life that live in these worlds? Aren't they adapted to such gases and other substances that are toxic to us? What is the most imporant thing? An extrasolar planet that we can live or simply the existence of extrasolar planets capable of develop complex life, even being toxic to us? Maybe many planets out there, in real life, will have such toxic gases in higher amounts than in our atmosphere. We are adapted to live in our kind of atmosphere. And aliens are adapted to live in other atmospheres.

Venus and Mars both have high concentrations of CO2. Earth and Titan, N2. Gas giants, H and He.

Criogenic rocky or icy planets (-210°C to -185°C) will tend to have more H, CH4, CO and even N2 in form of clouds. If the planet is small (like pluto or triton), it will only have a thin atmosphere of nitrogen and methane. If it's big enough it may have a thick hydrogen rich air, like gas giants, and if "warm" enough, will allow seas of liquid nitrogen with vast clouds of it. It will look like a snowball Earth!

Cold rocky or icy planets (-185°C to -15°C) will tend to have more N2, NH3, CH4, H or even CO or SH2 (if have much volcanic activity, like Io).

Temperate and tropical earthlike planets (-15°C to 100°C) will tend to have more N2, CO2, O2, H2O, NH3 and N2O (if the air pressure is high enough to keep ammonia liquid), SO2 or SH2 (if they have much volcanic activity). Or CH4 and other hydrocarbons and organics (if it's a carbon planet).

Extreme hot rocky planets (500°C to beyond) may have Na and even silica or iron vapor!

Planets that have high CO2 and SO2 are planets that have or had much volcanism and kept such gases. Planets with lots of N2 and O2, like Earth, had much ammonia and water transformed by organisms, ultraviolet and storms. Some more exotic planets may have higher amounts of Cl2, F2 and Br2. And gas giants will have much H2 and He because... they are mostly made of light gases.

First example: A hot rocky planet with sulfuric acid seas and clouds, with it's own biosphere with complex life, probably will have an atmosphere made mostly of N2, O2, SO2 (instead of CO2), H2O and H2SO4, and maybe will have Cl2 too, if it's halogen rich. Halogens in this planet will play a vital role in native life, such as hydrogen do on Earth. And also metals like iron and maybe even silicon!

Second example: A cold rocky planet with vast seas of ammonia, also with complex life, probably will have an atmosphere mostly made of N2, CH4 (instead of CO2), NH3 (instead of water), PH3, N2O and C2H2 (it would be their "oxygen") and some more hydrocarbons. But if it has seas of water mixed with ammonia, maybe will also have a lower amount of O2 and H2O. Metals like sodium, magnesium, calcium, rubidium, potassium and maybe cesium, and including boron, will be important to generate electricity that will be stored in cells like batteries, because life on colder planets will have to rely more on electricity, and because ammonia with water dissolve well these metals, electricity might be a viable source of energy for life.

Third example: A temperate rocky world with vast seas of hydrocarbons, such as gasoline. The vast seas of gasoline and it's own biosphere maintain an atmosphere mostly made of N2, C2H2 (their "oxygen"), ethane and propane (their other "oxygen"), methane obviously, maybe also butane, simple and complex organics derived from the reactions in the atmosphere, benzene, and CO or even SO derived from volcanism. It is said that more complex hydrocarbons are better for life than simple hydrocarbons, so a hot or temperate carbon planet might be better than a cold one.

Fourth example: A cold rocky world with vast seas of alcohol or formaldehyde, also with life. The air would be mostly N2, CH4 with minor amounts of O2 derived from reactions. But the oxygen will have to stay low, keeping a reducing atmosphere, so the oceans won't catch fire!

Anyway, planets "identical" to Earth might be more an exception than a role in the universe.

I will also ask the developer of Space Engine to add H2SO4 in hydrosphere and atmosphere classification, because hot planets, but not so hot and dry as Venus, might have H2SO4 seas and clouds too, maybe supporting a biosphere. N2 oceans are also a possibility. Not only that, but maybe even Fe and NaCl seas too, in hotter planets. Maybe even HCl and HF too, exotic and rare substances. Also PH3 for a new gas and alcohol for hydrosphere. Extrasolar planets might not only have gases that we are used to.

H2SO4 (sulphuric acid) is already in our own atmosphere and helps in cloud formation and the desintegration of statues. But, yes, we would probably want to include other possible forms of exotic life living in very different environments. Let's look at how liquids are seen in our own neighborhood.

Chart 1Probability distribution for occurrence of different liquids as a function of distance from a Sun-like star.The x-axis represents distance, and the y-axis represents probability (summed probability of surface and subsurface liquid). Note that the x-axis is a log axis.

In respiration we oxidize substances, but if it there was a way of respiration that uses reduction, so CO2 could be used as a fuel, even if the organism lives in an oxidized environment? With this we could have a planet that most of it's atmosphere is CO2, so the organisms will have plenty of fuel to use. Planets like Venus but with more favourable conditions, like earth-like temperature and the presence of liquid water or other solvent. I'm not talking about photosynthesis, but another reaction that uses CO2 and may produce O2, CH4, H2O or H2, in earth-like conditions, but with an atmosphere rich in CO2 and poorer in N2 or O2.

In respiration we oxidize substances, but if it there was a way of respiration that uses reduction, so CO2 could be used as a fuel, even if the organism lives in an oxidized environment? With this we could have a planet that most of it's atmosphere is CO2, so the organisms will have plenty of fuel to use. Planets like Venus but with more favourable conditions, like earth-like temperature and the presence of liquid water or other solvent. I'm not talking about photosynthesis, but another reaction that uses CO2 and may produce O2, CH4, H2O or H2, in earth-like conditions, but with an atmosphere rich in CO2 and poorer in N2 or O2.

The problem of reducing CO2 is that, in most cases, including the ones you give for producing CH4 and O2, the reaction would absorb energy rather than release it. The point of respiration is that it releases energy that an organism can use.

Thanks. I love the idea that anything is possible, but also like trying to keep it in the realm of physical possibilities. We humans understand the laws of nature, physics, math, chemistry, etc well enough and should be able to predict, with the help of computers, what possible ways energy transfer and information storage can function in a given environment and from there what chances life has of developing. It will no doubt be stranger than we can imagine but in hindsight it will make complete sense.